In commonly owned U.S. Pat. No. 4,197,007 there has been disclosed a device for intercepting and measuring back-scattered rays from the interior of an optical fiber, illuminated by a pulsed laser, for the purpose of determining its attenuation coefficient and locating possible faults. Semiconductor lasers generally used for such evaluation, however, have rectangular emitting surfaces whose radiation can be focused by conventional systems upon a circular fiber end only with a significant loss of energy and/or with a poor utilization of the numerical aperture of the fiber. With a uniform magnification ratio in planes parallel to the major and the minor sides of the laser face, as provided by the spherical lenses of the prior patent, the image of that face projected upon the fiber end (whose diameter may be considerably smaller than the height of the laser) will either fall partly outside the fiber end or be limited to a fraction of its diameter in the transverse direction.